Integrated Information Theory (IIT) proposes that consciousness is a fundamental property of systems capable of integrating information.

Developed by Giulio Tononi, Integrated Information Theory (IIT) is a leading mathematical framework that attempts to characterize consciousness based on information processing principles. IIT postulates that consciousness is identical to 'integrated information' (quantified by a metric called 'phi' or Φ), which is generated by a system that has both a rich repertoire of states (differentiation) and is irreducible to its individual parts (integration). In essence, a system is conscious if it has a unique cause-effect structure that cannot be broken down into independent components. IIT suggests that consciousness is not confined to biological brains but could theoretically arise in any sufficiently complex system that satisfies its axioms. The theory provides specific postulates about the phenomenology of consciousness (e.g., it is intrinsic, structured, specific, and integrated) and attempts to derive these from mathematical properties of information flow. While highly abstract and challenging to fully test, IIT offers a principled way to measure and compare the level of consciousness in different systems, from brain regions to artificial networks.